Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 241419

Area Forecast Discussion
National Weather Service Honolulu HI
419 AM HST Mon Apr 24 2017

A ridge of high pressure to the north of the islands will maintain
a trade wind flow into the middle of the week. Limited shower
activity is expected into Tuesday. From Wednesday and into the
weekend, increasing showers are expected, with the potential for
an extended period of unsettle weather.



The overnight clouds and showers did not quite formed. It is more
isolated this morning than what the official forecast has, that
is scattered windward and mountain showers. Barring any sudden
change to the amount of low clouds upwind of the islands, it looks
like it will be a rather dry Monday. High cirrus clouds are not
helping by masking the low level features over and upwind of the
islands. However, based on the last visual satellite picture of
yesterday at 5 pm HST, it looked rather dry upwind for several
hundred miles. Lending support to a dry morning, the Hilo sounding
this morning is a dry 0.73 inch precip water, including a stable
inversion. Lihue sounding is a tad more moist at 1.08 inch but the
low level moisture is capped by a strong inversion at 5k feet.
The GFS solution may have a lock on this as it keeps the low
inversion at 5k through Tuesday afternoon for the smaller islands.
Thus, we may be looking a a drier trade wind flow through Tuesday
than what are in the POP grids for Tuesday, that scattered showers.

The next change is slated to start on Tuesday night or Wednesday
with the inversion being lifted higher to 10k feet over the
smaller islands, and to 13k feet over the Big Island. This is the
beginning of the unsettled weather pattern, caused by a digging
and deepening upper level trough as it approaches the main
Hawaiian Islands from the NW. The rest of the forecast remains the
same, calling for an extended showery period, lasting to at least
next week Monday. Please refer to previous discussion below.

A clarification about the front as noted on the GFS since it is
not clear cut as the ECMWF solution. The front is slated to reach
Kauai Friday afternoon and Oahu Friday evening with some cooler
northerly winds behind it. The front and cool air then eases over
to Maui county Saturday where it will stall out. Both the ECMWF
and GFS are pretty much in sync with this scenario.

The clouds and showers eased off across Kauai, finally, and had
really nice sunset. The last visual satellite imagery, at 5 pm
HST, showed very scant low cloud coverage across the island`s
windward water...and this clearing extended upwind of the islands
for several hundred miles. Low cloud coverage will be increasing
as the evening progresses, however, and so will the showers. The
forecast was updated to reflect this scenario, and also to lower
the cloud coverage and POPs for Kauai in particular.

Dense cirrus clouds, at around 25k feet, have increased across
the eastern half of the island chain this afternoon and evening.
These clouds are expected to linger through the rest of the night
before retreating south and eastward Monday clearing Maui County
in the morning, and the Big island late in the afternoon.

The cirrus is being carried over the islands from an upper level
trough orientate NE to SW, and located just W of Kauai. A low is
forecast to form along the trough W of Kauai later tonight and
drift S to a position of 250 miles SW of Kauai late Monday
afternoon. Then the low turns into a deep trough by Wednesday.
During this period, the main surface feature will be the surface
high of 1026 mb located 1200 miles NE of the main Hawaiian
Islands. The associated ridge extends westward from the high
passing some 450 miles N of Kauai. Through Tuesday...the ridge
will be hold its ground while a front tries to advance eastward.
Trades, therefore, will continue across the Hawaiian Islands.

Thereafter, the forecast may start to turn sour. An upper level
trough with a pool of unstable cold air is forecast to drop down
onto the main Hawaiian Islands Thursday night. The air mass will
start destabilizing across the area as early as Wednesday night
leading to widespread showers, a few locally heavy including a
slight chance of thunderstorms. Various part of the island chain
will be affect by this weather system. The surface ridge breaks
down and the front advances toward the islands, reaching Kauai
early Friday morning as per ECMWF solution. The GFS has a slightly
different picture with the formation of a N to S oriented low
level trough over Maui County while the front passing just N of
Kauai. But any way you look at it, the trade flow will be
disrupted and there will be some significant weather around as
early as Thursday night.

The weather systems lingers into the upcoming weekend and into
next week Monday. A surface low may even form along the surface
trough near the islands thereby enhancing some of the showers.
Both the ECMWF and GFS are in on this. More information will be
proved in the coming days should the models continue on this very
wet outlook.


High pressure far northwest of state will persist. Mostly dry
trade winds will focus ragged clouds and scattered showers along
windward slopes and coasts. Isolated MVFR ceiling are possible in
passing showers otherwise VFR conditions will prevail all areas.

An upper level trough about 450 miles west of Oahu will continue
to generate bands of higher clouds...mainly over the eastern half
of the state. This cirrus layer, combined with an increasingly
stable air mass, will limit any convective development along the
leeward slopes of the Big Island Monday afternoon.

The subsidence inversion over the state ranges from about 7000 ft
over Kauai to around 5000 ft over the Big Island. This, combined
with increasingly brisk summit level winds could generate some
low level turbulence to the west of terrain on the Big Island and
Maui on Monday. Conditions are expected to be remain isolated, so
no AIRMET anticipated at this time.


The latest ASCAT pass showed Small Craft Advisory (SCA) winds
still over the coastal waters around the Big Island, but below
SCA winds around Maui County. Therefore, the Windward Maui County
waters and Pailolo Channel have been dropped from the advisory.
The rest of the waters around the Big Island remain under the SCA
through Monday night due to strong ESE winds. Extension of the SCA
may be needed into Tuesday if wind speeds remain elevated.

A number of mainly small swells will affect the state this week,
but High Surf Advisories are not expected for at least the first
half of the week. Wind waves, along with a little bit of a trade
wind fetch upstream, will bring small, but choppy waters across
the east facing shores through most of the week. Small north and
northwest swells will fade in and out over the next week as well.
Small, long period south swells will also fade in and out this
week, but will need to be monitored for possible advisory level
surf along south facing shores toward the end of this week.

Small Craft Advisory until 6 AM HST Tuesday for the Alenuihaha
Channel, windward Big Island Waters, and waters south of the Big



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